A DC-DC converter converts an input voltage and an input current, to output a constant voltage and a particular current value. The current output from the DC-DC converter to a load may be reduced and in this case, degradation in efficiency may occur. Such a phenomenon may be caused by the propagation delay of a signal generated from a pulse width modulation (PWM) comparator included in the DC-DC converter (hereinafter, referred to as a “PWM” signal).
FIG. 1A depicts waveforms of a PWM signal and a current IL flowing through a load in a high load state. FIG. 1B depicts waveforms of the PWM signal and the current IL flowing through the load in a low load state. FIG. 1C depicts waveforms in the case in which the PWM signal is skipped in the low load state.
As the PWM signal is delayed (tDelay), the ON time of an internal power transistor increases. As a result, a load current (indicated by a hatched portion) is also increased which, in turn, increases power consumption. This is called “switching loss”.
The “high load state” refers to a state in which a large current flows through the load as the internal power transistor is turned on. The “low load state” refers to a state in which a small current flows through the load. In the low load state, the influence of the switching loss is more than that in the high load state. In the low load state, accordingly, the efficiency of the DC-DC converter is further reduced due to the increased switching loss.
The ON or OFF time of the internal power transistor may be determined in accordance with the frequency of the PWM signal to control the internal power transistor. As the ON time of the internal power transistor increases, the current flowing through the load is increased. For example, in the case of a DC-DC converter for an AMOLED, the DC-DC converter may operate in a low load state due to high-frequency operation thereof. In such a low-load operation, the influence of switching loss caused by the delayed ON time of the internal power transistor in the DC-DC converter is very high. Meanwhile, in the low load state, a long discharge time occurs due to a small discharge amount of current, as shown in FIG. 1B. Furthermore, in the low load state, the delay time tDelay of the PWM comparator influences the discharge time. As a result, there may be problems in that the ripple of the load current IL increases, or the pulses of the PWM signal may be skipped.